A senior Syrian government official says the regime was willing to negotiate the exit of embattled Syrian president Bashar al Assad, as violence escalates on the streets of Damascus and Aleppo.

At least 12 people were killed in a raid on a district of the capital, while fighter jets and artillery pummelled the northern city of Aleppo and rebels claimed seizing parts of a town on the Iraqi border.

The United States and France again pushed for Mr Assad to stand down after deputy prime minister Qadri Jamil said Damascus was ready to discuss Mr Assad's departure as part of a negotiated settlement to end the 17-month conflict.

"As far as his resignation goes - making the resignation itself a condition for holding dialogue means that you will never be able to reach this dialogue," Mr Jamil said during a visit to Moscow.

"Any problems can be discussed during negotiations. We are even ready to discuss this issue."

The Syrian opposition, however, has ruled out any dialogue unless Mr Assad leaves power.

Political sources in Damascus, speaking on condition of anonymity, said that while in Moscow, Mr Jamil discussed the possibility of organising an internationally supervised presidential election open to all candidates, including Mr Assad.

But Mr Assad's candidacy has been rejected by the US, European countries and several Arab states.

Moscow again told the West not to meddle in Syria after US president Barack Obama hinted at possible military action if Damascus resorted to its chemical weapons arsenal.

"There should be no interference from the outside," Russian foreign minister Sergei Lavrov said after his talks with Mr Jamil.

"The only thing that foreign players should do is create conditions for the start of dialogue."

But the US was unimpressed by the apparent opening from Damascus.

"We still believe that the faster Assad goes, the more chance there is to quickly move on to the day after," State Department spokeswoman Victoria Nuland said.

Thomas Pierret, a lecturer in contemporary Islam at the University of Edinburgh, said Mr Jamil's comments were a "delaying tactic".

"The regime has opted for a military solution and will not change until it falls."

Former chief of the opposition Syrian National Council, Burhan Ghalioun, also dismissed Mr Jamil's comments.

"Every time the regime wants to buy time, it calls for dialogue," he said.

"It doesn't think for a moment to stop the war on the people. If it were serious about dialogue, it would stop the war."

'Long' battle continues

The violence on the ground showed no let-up on Wednesday, after a bloody day that saw at least 198 people killed nationwide, according to the Syrian Observatory for Human Rights.

Fighter jets hit a rebel-held neighbourhood of the northern city of Aleppo and shelled several other areas of the city that has become the key battleground since fighting erupted there a month ago, it said.

Residents said air strikes around the towns of Marea and Tal Rifaat, north of Aleppo, could be heard across the city.

Army forces also staged a massive assault on parts of Damascus and the nearby countryside, killing at least 20 civilians and leaving many fearful residents trapped in their homes, the observatory said.

Twelve people were also killed by troops in a raid on a Damascus district, the British-based observatory said, a day after it reported dozens killed when regime forces stormed another suburb of the capital and allegedly attacked a funeral procession.

It also said rebel fighters had seized control of an intelligence office and checkpoints in the eastern town of Bu Kamal on the Iraqi border.

The Observatory, which put Wednesday's nationwide death toll nationwide at 77, has a network of activists on the ground but its claims cannot be independently verified.

Syrian forces appear to be increasingly resorting to attacks from the air, particularly in the Aleppo area, as the rebels continue to put up stiff resistance on the ground despite the regime's far superior military might.

A top commander of the rebel Free Syrian Army, Colonel Abdel Jabbar al-Okaidi, said the rebels now controlled 60 per cent of Aleppo, but a security source in Damascus dismissed the claims.

"Reinforcements from both sides are heading to Aleppo," the source said.